Log periodic antennas operate over a broad frequency range. Generally log periodic antennas have a plurality of dipole elements in a planar spaced array. The length of the elements and the spacing between the elements are selected in accordance with a mathematical formula, with the shortest elements being near the top of the antenna. Feed conductors generally connect at the tip of the antenna. Electrical connections from feed conductors to opposed elements are alternated to provide a 180 degree phase shift between successive elements.
U.S. Pat. No. 5,093,670 to Braathen discloses a log periodic antenna formed by printed circuit board manufacturing methods onto an insulative substrate. The dipole elements and one feed conductor are formed on one side of the substrate and a second feed conductor is formed on the opposite side of the substrate. Vias though the substrate connect the second feed conductor to alternating opposed dipole elements.
U.S. Pat. No. 5,917,455 to Huynh et al. discloses an array of log periodic antennas mounted on a backplane. Each antenna includes two flat dipole strips of conductive material with bases of the dipole strips mounted to the backplane in a spaced configuration. Each antenna is fed by a coaxial feed line with the center conductor being connected to one dipole strip and the jacket conductor being connected to the other dipole strip.
U.S. Pat. No. 6,133,889 to Yarsunas et al. and U.S. Pat. No. 6,243,050 to Powell disclose antennas with log periodic dipole assemblies fed by a microstrip feed line. Each dipole assembly has two flat dipole strips of conductive material with the bases of the dipole strips being mounted to a backplane in a spaced configuration. The feed line extends between the dipole strips of a dipole assembly and is connected to one dipole strip of the dipole assembly with a connector either at the top of the dipole strip or intermediate the top and the base of the dipole strip. The other dipole strip of the dipole assembly is not connected to the feed line.
The “diode junction effect” can be caused by metal to metal junctions, such as welded, soldered, riveted or bolted junctions, in electronic circuitry. This “diode junction effect” creates a non-linear voltage-current characteristic that, in radio frequency (RF) signals, can create intermodulation products that are different than the original frequencies. Passive intermodulation (PIM) may manifest as relatively strong interference signals. It is therefore desirable to avoid metal to metal junctions between the feed line and the tip of a log periodic dipole antenna, and in the feed line to the antenna.